Device and process for the reversible storage of electric energy by its reversible conversion to kinetic energy

ABSTRACT

In the device, an electric motor is designed as a brushless, electronically commutated pancake motor (201) with permanent magnets arranged on the rotor and coreless windings arranged on the stator. Each winding is provided with a pair of terminals which can be connected in each case via a controllable switch at least to each phase conductor of the alternating power supply and to each other via a further controllable switch. 
     In a device with pick-ups, the instantaneous voltage between one phase conductor each and a neutral conductor or another phase conductor of the alternating power supply, the instantaneous voltage between the two terminals of at least one pair of terminals, and also the instantaneous current in one winding each are repeatedly sampled. The sampled instantaneous voltages or currents are compared with one another and with corresponding threshold values in order to control the switches in dependence on the instantaneous result of the comparisons. 
     The device allows a reversible conversion and storage of electric energy from an a.c. power supply into kinetic energy of an electric motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for the reversible storage of electricenergy by its reversible conversion into kinetic energy in an electricmotor, the electric energy being drawn from an a.c. power supply ofpredetermined frequency and fed back into the a.c. power supply, andalso to a process for the operation of this device and to use of abrushless, electronically commutated pancake motor with permanentmagnets arranged on the rotor and coreless windings arranged on thestator as an energy store in the abovementioned reversible storage ofelectric energy.

2. Discussion of the Prior Art

Unless otherwise specified, it is generally to be understood inconjunction with the present invention that the kinetic energy stored inan electric motor may be present both in the mechanical form and in theelectrical form or as a combination and sum of these two forms ofenergy.

The mechanical form of the kinetic energy stored in the electric motoris stored in the rotation of the motor. The electrical form of thekinetic energy stored in the electric motor is stored in the currentwhich flows through those parts of the stator winding which act as aninduction coil or choke. It is well-known to a person skilled in the artof mechanics that the rotating rotor stores a mechanical form of kineticenergy dependent on its inertia and its rotational speed, and it iswell-known to a person skilled in the art of electrical engineering thata choke or induction coil stores an electrical form of kinetic energydependent on the current flowing through it (for further explanation itis submitted that, in contrast to this, the electric energy in acapacitor is stored in the form of potential energy, which has theeffect that in an LC oscillator the electric energy is exchanged betweenthe potential form and the kinetic form of electric energy). Inconjunction with the present invention, the mechanical form and theelectrical form of kinetic energy are to be regarded and consideredtogether.

In particular for the uninterrupted feeding of a load from an a.c. powersupply, it is known to use as the energy store for the reversiblestorage of electric energy a d.c. motor which is connected via asupply-side converter to the a.c. power supply and via a load-sideconverter to the load. In the case of such devices it is alwaysnecessary to provide a change-over facility, in order if need be, i.e.depending on the conditions prevailing at the a.c. power supply and atthe load, to connect the load to the a.c. power supply or to theload-side converter, or to change it over. In this context it is alsoknown to correct shortcomings of the a.c. power supply, such asdeviations or distortions of voltage, waveform, frequency, phase etc.,and reactions of the load on the a.c. power supply by electric energy,which is taken from the d.c. motor and fed via the supply-side converterand/or the load-side converter into the a.c. power supply or into theload, respectively.

There are, however, some cases in which this known technique cannot beused at all, or only to a restricted extent or with reservations, inparticular for reasons of cost and/or for reasons of space. Such casesare to be encountered in drive technology, if very fast starting timesand braking times (the latter for example for accurate positioning) arerequired, such as in lifts, conveyor belts, electric motor cars, robotsetc.

SUMMARY OF THE INVENTION

Therefore it is an object of the invention to propose in the case of adevice for the reversible storage of electric energy a solution which ismore favourable than hitherto with respect to costs and spacerequirement.

To achieve this object, a device for the reversible storage of electricenergy of the type mentioned at the beginning is characterized by thecombination of features specified in claim 1. A process for theoperation of this device is characterized by the combination of processsteps specified in claim 7. Advantageous further developments of thedevice and of the process are defined in the dependent claims.

In conjunction with the invention it is to be understood that thesampling of the various voltages and currents takes place with arepetition frequency which is substantially greater than the frequencyof the a.c. power supply, in the case of an a.c. power supply of 50 Hz(for example in the case of industrial current) or 400 Hz (for examplein the case of aircraft) typically between 20 kHz and 100 kHz. Althoughfrequency values below 20 kHz can also be used, they may be below theupper audible limit of the human ear, which can have unpleasantconsequences. Frequency values above 100 kHz can also be used as well,but the technical problems in realising the circuits become more andmore difficult to solve with increasing frequency values.

Also within the scope of this invention is the use of a brushless,electronically commutated pancake motor with permanent magnets arrangedon the rotor and coreless windings arranged on the stator as an energystore in the reversible storage of electric energy by its reversibleconversion into kinetic energy in a pancake motor, the electric energybeing drawn from an a.c. power supply of predetermined frequency and fedback into the a.c. power supply. A brushless, electronically commutatedpancake motor is known per se, for example from the article by M. Salamiin Antriebstechnik 30/8 (1991), and is commercially available. Inconnection with the permanent magnets arranged on the rotor of thepancake motor and the coreless windings arranged on the stator of thepancake motor, reference is also made to patent documents U.S. Pat. No.4,187,441, JP-61-185050, JP-01-253211, DE-2143752 and/or DE-2345150.

By the invention it is possible in the case of drive units for hybridelectric motor cars to form a very efficient and compact drive unit withthe device according to the invention, a storage battery, capacitors anda combustion engine, which with existing technology results in a vehiclewhich is too heavy.

It is also possible by the invention to start up jammed conveyor beltsand the like with a brief very strong output, in particular if thejamming is attributable to friction when at a standstill.

Moreover, by the invention it is possible in the event of a powerfailure still to move a lift and the like to the next floor or to thelowermost floor.

In addition, by coupling the energy store designed as an electric motorto a mechanical drive, the invention allows, for example, small heatingsystems for blocks of houses to be provided with the possibility, ataffordable costs, of acting as small power stations and consequentlyserving in the event of power failure as an emergency emergency powergenerating set. Thus, as well as emergency power generating sets, smallpower stations driven by wind or water are made smaller and moreefficient, in particular the emergency power generating sets can also beused as waveform modifiers and for phase correction for the a.c. powersupply.

The essence of the invention is the direct and reversible conversionbetween kinetic energy stored in an electric motor and electric energyof any desired voltage or frequency. In principle, the winding of theelectric motor is used directly as a storage choke, so that it becomespossible in a double conversion (i.e. carried out one way and back theother way) to convert alternating current directly into alternatingcurrent without the diversion via direct current and converters. Thisresults in a significant saving in costs and weight.

In addition, the winding of the electric motor can bear brief overloadsor supply overcurrents which the converters would not bear. Therefore,the electric motor need not be designed for high peak output values, butessentially only for their average values with a much lower ratio ofpeak output to output in continuous operation than in the case of thepreviously used systems with electric motors and converters. Thisresults in a further significant saving in costs and weight.

The kinetic energy stored in the rotor of the electric motor increasesin proportion to the square of the rotational speed of the rotor, sothat the use according to the invention of a brushless, electronicallycommutated pancake motor with permanent magnets arranged on the rotorand coreless windings arranged on the stator results in a significantsaving in costs and weight for the same output. Such a pancake motor isnamely capable of running at very high rotational speeds, and itscontrol is facilitated by its rotor having a low weight and a lowinertia and therefore responding to the supply or removal of electricenergy with great differences in rotational speed.

The device according to the invention and its method of operation can beoptimized for various operating modes, for example for constant speed orconstant torque or greatest-possible torque at the motor shaft,best-possible quality or least-possible reactions on the a.c. powersupply (with respect to deviations or distortions of voltage, waveform,frequency, phase etc.), greatest-possible output, greatest-possibleefficiency, etc.

In the case of phase-shifting operation, the device according to theinvention and its method of operation are of interest in particular inconnection with the use of superconductors, since the majority of alllosses occurring in the pancake motor occur in the ohmic resistance ofthe coreless windings arranged on the stator. This heat loss must bedissipated, which at the low temperatures of the superconductors isgenerally very complex, but in the case of a pancake motor with corelesswindings arranged on the stator is far less complex than in the case ofthe conventional electric motors.

In an operating mode for achieving the greatest-possible output, it isendeavoured to convert as much energy as possible reversibly between theelectrical form and the kinetic form. This corresponds to a malfunctionand/or emergency operation, which tends to be a rare occurrence and isgenerally followed by a lengthy cooling phase. For example, it concernssituations where a lift becomes stuck due to overloading and it isrequired to continue its movement to the next floor, or situations inwhich, after being at a standstill for a long time, a conveyor belt isstuck by adhesive, resin or hardening of the rubber parts and has to bebrought into motion with a jerk, or else situations where, in the caseof a wind power station, a gust continues until it has the effect ofcorrespondingly altering the direction or the angle of pitch of therotor. Here, the output is limited by the maximum permissible values:maximum permissible current in the power supply and in the devices forthe electronic commutation of the pancake motor, maximum permissibletemperature of the windings in the stator of the pancake motor. Acceptedhere are the distortions of phase and waveform (such as current peaks)occurring on account of the reactions on the a.c. power supply, and alsointerfering radiations, these interferences generally not being greaterthan those which are caused by storms, arcing, switching on or switchingoff of heavy-duty equipment, earth faults and the like and ought inprinciple to be withstood by the power supply.

In an operating mode for achieving the greatest-possible braking effect,it is endeavoured to consume as much kinetic energy as possible byconverting it into the electrical form or into heat. The latter isclearly a case of emergency operation. In this operating mode, thepancake motor is normally transformed into a generator, and the electricenergy thus generated is fed back into the a.c. power supply. If sodesired, the generated electric energy may be fed via current conductorsof an auxiliary power supply into a battery or be converted into heat ina resistor. If, however, the fault is a power supply failure, it is notpossible to feed the generated electric energy back into the a.c. powersupply, since on the one hand the power supply voltage drops to zero andconsequently the power supply does not consume any output, on the otherhand this feeding back is generally prohibited by regulations. In such aspecial case, in which the return of energy is not possible, thecontrolling of the switches may be performed in such a way thatconditions which are as unfavourable as possible with regard toefficiency are created. For example, it can be arranged that one windingof the rotor generates a current which is fed with "reversed" polarityto another winding, in order to counteract the rotating field in thepancake motor, i.e. one winding brakes and the other winding drives,which in a normal case would be senseless. The kinetic energy is thenlost as heat in the dissipative resistances of the windings of thepancake motor and in the switches (semiconductor switching elements suchas IGBTs). Of course, the elements thus heated require a correspondingcooling break until their operation may be resumed. For example, suchspecial cases are situations where the current fails when a lift ismoving and the invention make it possible nevertheless to continue themovement to the next floor with a braking action, or situations in whicha conveyor belt must be stopped gently or abruptly in the event of apower failure, or it is required that an electric motor car is stillcapable of braking after the battery fuse has blown or the main switchhas been switched off, the emergency feeding of the antiblocking systemmaintaining its effectiveness, or else situations where in the case of awind power station the occurrence of a gust requires electric energy tobe consumed as heat, in order that the rotor is not accelerated toinadmissibly high speeds. In an operating mode for achievingleast-possible reactions on the a.c. power supply (least-possibledeviations or distortions of voltage, waveform, frequency, phase, etc.),the controlling of the switches may be performed in such a way thatwinding parts act as storage chokes in order to consume or supply powersurges, which consequently are not consumed or supplied by the a.c.power supply. It is accepted here that energy is lost as heat in thedissipative resistances of the windings of the pancake motor and in theswitches (semiconductor switching elements such as IGBTs). In thisoperating mode, the device according to the invention does not respondas quickly as possible to voltage deviations, the pancake motor behavesas if connected with high impedance to the power supply. Consequently,no high currents occur even in the event of sudden fluctuations of thevoltage and/or of the frequency. In isolated operation with a pluralityof small power stations combined, the coupling-in of the deviceaccording to the invention with the pancake motor acting as a generatoris easy to accomplish, the power supply feed line does not need towithstand any overcurrent values and can be designed to correspondinglylow requirements. It is accepted here that operation does not take placewith the best-possible efficiency; corresponding energy losses andheating occur.

In an operating mode for achieving best-possible quality of the a.c.power supply with respect to deviations or distortions of voltage,waveform, frequency, phase etc., the device according to the inventionis "hard"-connected to the a.c. power supply, i.e. every deviation ordistortion is counteracted as quickly as possible. If, for example, aconsuming unit generates an inductive reactive load, the deviceaccording to the invention compensates for the corresponding deviationsor distortions within its (naturally limited) capabilities. Thiscompensation results in mechanical deviations or distortions of torqueand rotational velocity and generates corresponding forces, which aretransferred to the stator of the pancake motor and are to be absorbed byits housing. It is accepted here that commands of a ripple control inthe power supply are to be detected and protected against the effect ofthe device according to the invention, since the device according to theinvention would otherwise interpret the commands of the ripple controlas interferences and would eliminate them as best possible. If sodesired, the electric energy corresponding to the interferences may bebuffered by a battery via current conductors of an auxiliary powersupply, to be precise in the case of relatively small outputs almostuntil the rotor of the pancake motor is at a standstill. Of particularinterest in this context is the fact that the device according to theinvention allows small heating systems for blocks of houses to beprovided with the possibility of acting as small power stations andconsequently, in the event of a power failure, of serving as anemergency power generating set: if such small power stations areinterconnected in sufficient number, an emergency power supply to entireline sections over a certain time also becomes possible. It likewisefollows that, due to the reduction of the reactive power in the powersupply, the losses in the lines are reduced, which improves thestability of the supply voltage at remote locations with weak feedlines.

Normal operation of the device according to the invention is to be seenin a compromise between the abovementioned, partly conflicting functionsand properties.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in more detailbelow with reference to the drawing, in which:

FIG. 1 shows an electrical block diagram of an exemplary embodiment ofthe device according to the invention, and

FIG. 2 shows an electrical block diagram of a further exemplaryembodiment of the device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, a the block 101 diagrammatically designates abrushless, electronically commutated pancake motor. As alreadymentioned, such a pancake motor is known per se, as is its configurationwith permanent magnets arranged on the rotor and coreless windingsarranged on the stator. For this reason, this pancake motor need not bedescribed in any more detail here. In the design according to FIG. 1,this pancake motor 101 is of a three-phase design and is provided withthree windings, which are diagrammatically represented and are denotedby 102. The individual windings 102 are each provided with an assignedpair of terminals, which are denoted by 112 and 122.

It is to be understood that the three-phase design of the pancake motor101 with three phases U, V, W is given as an example, since the pancakemotor 101 may be provided with any desired and expedient number ofphases.

Continuing to refer to FIG. 1, a three-phase a.c. power supply 103 witha frequency of 50 Hz is diagramatically represented by three phaseconductors 104 and a neutral conductor 105.

Here too, it must be understood that the three-phase design of the a.c.power supply with three phases R, S, T and a neutral conductor N at afrequency of 50 Hz is given as an example, since the a.c. power supplymay be provided with any desired and expedient number of phases and beoperated at any desired and expedient frequency, for example even at 400Hz.

Moreover, in the case of the exemplary embodiment of the inventiondescribed here, there is no relationship between the number of phases ofthe pancake motor 101 and the number of phases of the a.c. power supply103.

Each terminal 112 and 122 of each winding 102 can be connected via anassigned controllable switch 114, 124, 134 to each of the phaseconductors 104 of the a.c. power supply 103 and also to the neutralconductor 105. Moreover, the two terminals 112 and 122 of each winding102 can be connected to each other via an additional assignedcontrollable switch 115.

Referring to FIG. 2, the block 201 diagrammatically designates thereinthe brushless, electronically commutated pancake motor. For betterillustration of the invention by a different exemplary embodiment, thispancake motor 201 is of a two-phase design here, with two phases U, V,and is provided with two windings, which are diagrammaticallyrepresented and are denoted by 202. The individual windings 202 are eachprovided with an assigned pair of terminals, which are denoted by 212and 222.

Continuing to refer to FIG. 2, a three-phase a.c. power supply 203 witha frequency of, for example, 50 Hz or 400 Hz is diagrammaticallyrepresented by three phase conductors 204 without neutral conductor.

This makes it clear that also in the case of the exemplary embodiment ofthe invention described here there is no relationship between the numberof phases of the pancake motor 201 and the number of phases of the a.c.power supply 203.

Each terminal 212 and 222 of each winding 202 can be connected via anassigned controllable switch 214, 224 to each of the phase conductors204 of the a.c. power supply 203. Moreover, the two terminals 212 and222 of each winding 202 can be connected to each other via an additionalassigned controllable switch 215. The switches 214, 215, 224 are part ofa complete group of switches 216, but they are controlled individuallyby a device 217 which is substantially composed of a microprocessor orat least comprises a microprocessor.

In a group 240 of measuring devices, pick-ups 241, 242, 243 are providedfor the voltage between the two terminals 212, 222 of the respectivepairs of terminals and for the current of the respective winding. Thesepick-ups are driven by the device 217 in such a way that a periodicallyrepeated sampling of the relevant voltages and currents takes place. Asalready described, the frequency of this sampling is substantiallygreater than the frequency of the a.c. power supply and, in the case ofan a.c. power supply of 50 Hz (for example in the case of industrialcurrent) or 400 Hz (for example in the case of aircraft) typically liesbetween 20 kHz and 100 kHz.

Likewise provided in the group 240 of measuring devices drivenperiodically by the device 217 are pick-ups 244 for the voltage at thephase conductors 204 with respect to another phase conductor 204 of thea.c. power supply 203 or, with respect to a neutral conductor (not drawnin).

Apart from the microprocessor mentioned, the device 217 also comprisesdevices which allow the sampled instantaneous voltages or currents to becompared with one another and/or with corresponding threshold values andallow the switches mentioned to be controlled in dependence on theinstantaneous result of these comparisons. Of course it must beunderstood that these comparing devices and controlling devices or partsthereof may also be formed in the microprocessor and that their functionmay be executed by the microprocessor.

The switches 114, 115, 124, 134 mentioned in conjunction with FIG. 1 arepart of a complete group of switches 116 and are likewise controlledindividually by a device of the type described: however, in FIG. 1 thisdevice is not drawn in in order to simplify the representation.Moreover, in the exemplary embodiment represented in FIG. 1 there arelikewise provided measuring devices and pick-ups of the same type as inthe case of the exemplary embodiment represented in FIG. 2, but in FIG.1 these measuring devices and pick-ups are also not drawn in in order tosimplify the representation.

The major difference between the exemplary embodiments of the inventionaccording to FIGS. 1 and 2 is that, in the exemplary embodimentaccording to FIG. 2, an auxiliary power supply 250 with currentconductors 251 is provided. Each terminal 212 and 222 of each winding202 can be connected via an assigned controllable switch 254 to each ofthe current conductors 251 of the auxiliary power supply 250. Theswitches 251 are also part of the complete group of switches 216, andthey too are controlled individually by the already mentioned device217.

In the exemplary embodiment according to FIG. 2, the auxiliary powersupply 250 comprises two current conductors 251, which are eachconnected to a terminal 252 of a store 253 for electric energy. In theexemplary embodiment according to FIG. 2, this store 253 for electricenergy is designed as capacitor 255, so that the two current conductors251 of the auxiliary power supply 250 are connected to each other interms of circuitry via this capacitor 255. However, it must beunderstood that the store 253 for electric energy may also be designedin some other way, for example as a storage battery.

Likewise provided in the group 240 of measuring devices drivenperiodically by the device 217 are pick-ups 264 for the voltage and, ifappropriate, the current at the current conductors 251 of the auxiliarypower supply 250 with respect to another phase conductor 251 of theauxiliary power supply 250 or with respect to a neutral conductor (notdrawn in) of the auxiliary power supply 250.

To make it easier to maintain an overview during the description whichnow follows of the process for operation of the device described above,it is pointed out that the device according to the invention isgenerally operated in such a way that, at appropriate times when thestates of the various voltages and currents prevailing at that instantallow,

a) energy is drawn from the power supply and stored in the electricmotor, or

b) energy is drawn from the electric motor and fed into the powersupply, or

c) no energy flows between the electric motor and the power supply, itbeing so if appropriate that

d) energy is drawn from the electric motor and fed into the energy storeconnected to the auxiliary power supply, or

e) energy is drawn from the energy store connected to the auxiliarypower supply and is stored back in the electric motor, or

f) no energy flows between the electric motor and the energy storeconnected to the auxiliary power supply.

For this purpose, the following take place with the already mentionedrepetition frequency and by means of the already mentioned measuringdevices:

a periodic sampling of the instantaneous voltage at the phase conductorsof the a.c. power supply with respect to the neutral conductor oranother phase conductor with subsequent determination of the phaseconductors of which the instantaneous voltage has the greatestinstantaneous positive or negative value in absolute terms;

a periodic sampling of the instantaneous voltage at each winding betweenthe corresponding two terminals with subsequent determination of whichterminal of the pair of terminals is positive or negative at thatinstant; and

a periodic sampling of the instantaneous current in each winding withsubsequent comparison of the absolute value of this instantaneouscurrent with a threshold value.

The threshold value mentioned above for the current in each windingcorresponds substantially to the nominal value which corresponds to thetype of the electric motor and is specified by the motor manufacturer.As a departure from this, the threshold value mentioned may also be setto a higher value, for example up to the maximum permissible currentwhich corresponds to the type of the electric motor. Such a highthreshold value may be used, for example, whenever the microprocessor isprogrammed in such a way that it allows the corresponding current toflow only briefly through the windings of the electric motor and anadequate cooling of the electric motor is ensured.

It is to be understood in this case, that, with respect to the periodicsampling of the instantaneous voltage at each winding between thecorresponding two terminals, either an actual measurement may beperformed, or the relevant voltage values may be calculated by themicroprocessor, for example on the basis of data from an absoluteshaft-angle encoder for the instantaneous position, rotational speed androtational acceleration of the rotor in the electric motor and on thebasis of input tables of possible values for the state of the electricmotor at any one instant. This possibility mentioned second is much lesscomplex than the first, but is nevertheless sufficient for achieving theobject of the invention.

On the basis of the results of the samplings, determinations andcomparisons specified above, the device described above (device 217 inthe case of the exemplary embodiment represented in FIG. 2) isprogrammed in order to execute the following process steps and makedecisions:

If the absolute value of the instantaneous current in a winding becomesless than the threshold value, actuation of the switches forinterrupting the existing connections of the two terminals of thecorresponding winding and for establishing a connection of the terminalwhich is positive at that instant to the phase conductor which has thegreatest instantaneous positive voltage value in absolute terms, and ofthe terminal which is negative at that instant to the phase conductorwhich has the greatest instantaneous negative voltage value in absoluteterms; and

if the absolute value of the instantaneous current in a winding becomesequal to or greater than the threshold value, actuation of the switchesfor interrupting existing connections of the two terminals of thecorresponding winding and for establishing a connection of these twoterminals to corresponding terminals of a device for the conversion ofenergy stored in the electric motor in kinetic form into another form ofenergy.

In a first exemplary embodiment of the device mentioned for theconversion of energy stored in the electric motor in kinetic form intoanother form of energy, this conversion device comprises quite simplythe winding and a short-circuit, i.e. the relevant energy conversion isbrought about by establishing a connection between the two terminals ofthe corresponding winding by means of the additional switch describedabove. As a result, in the electric motor mechanical energy (inconjunction with the rotation of the rotor) is converted into electricenergy (in conjunction with the current through the winding), thekinetic form being retained. Although ohmic losses also occur, they donot have substantial effects in the case of the process according to theinvention, so that the irreversible conversion of kinetic energy intoheat is not significant and is not discussed here in any further detail.The current through the winding then influences the rotation of therotor of the electric motor, and consequently the current through theother windings of the electric motor, in the way required by themicroprocessor, which has an effect on the exchange of energy betweenthe a.c. power supply and the electric motor.

In another exemplary embodiment, a connection between the two terminalsof the corresponding winding which is not directly to each other via theadditional switch, but via one switch each to one and the same phaseconductor of the a.c. power supply leads to precisely the same result.Here too, the conversion device mentioned comprises quite simply thewinding and a short-circuit.

In yet another exemplary embodiment, a connection between the twoterminals of the corresponding winding not directly to each other or toone and the same conductor of the a.c. power supply, but via one switcheach to one phase conductor each of the a.c. power supply leads to ananalogous result. As a result, energy is fed back from the electricmotor into the a.c. power supply, which indeed represents an exchange ofenergy between the a.c. power supply and the electric motor. Theconversion device mentioned substantially comprises here the winding andthe generator, which supplies the electric energy to the a.c. powersupply. In this exemplary embodiment, the device described above (device217 in the case of the exemplary embodiment represented in FIG. 2) ispreferably programmed to execute the following process steps and makedecisions on the basis of the results of the samplings, determinationsand comparisons specified above:

determination of an absolute value of the instantaneous voltage betweenpairs of phase conductors of the a.c. power supply;

determination of a pair of phase conductors of the a.c. power supply towhich a least absolute value of the instantaneous voltage corresponds:

connection of one of the two terminals each to one phase conductor eachof the pair thus determined, and this preferably by connection of theterminal which is positive at that instant to the phase conductor whichis positive at that instant and of the terminal which is negative atthat instant to the phase conductor which is negative at that instant.

In the case of the exemplary embodiment represented in FIG. 2, in whichan auxiliary power supply is provided, the device 217 described above isprogrammed to execute the following process steps and make decisions onthe basis of the results of the samplings, determinations andcomparisons specified above:

Sampling of the instantaneous voltage between a pair of currentconductors of the auxiliary power supply and comparison of an absolutevalue of this instantaneous voltage with the absolute value of theinstantaneous voltage between the two terminals of a winding;

if the absolute value of the instantaneous current in a winding becomesless than the threshold value,

and if the absolute value of the instantaneous voltage between the twoterminals of the relevant winding becomes equal to or greater than theabsolute value of the instantaneous voltage between the twocorresponding phase conductors of the a.c. power supply,

actuation of the switches for interrupting existing connections of thetwo terminals of the relevant winding and for establishing a connectionof that terminal of the corresponding winding which is positive ornegative at that instant to the corresponding positive or negativecurrent conductor of the auxiliary power supply,

and if, on the other hand, the absolute value of the instantaneousvoltage between the two terminals of the relevant winding becomes lessthan the absolute value of the instantaneous voltage between the twocorresponding phase conductors of the a.c. power supply,

actuation of the switches for interrupting existing connections of thetwo terminals of the relevant winding and for establishing a connectionof that terminal of the relevant winding which is positive at thatinstant to the phase conductor which has the greatest instantaneouspositive voltage value in absolute terms, and a connection of thatterminal of the relevant winding which is negative at that instant tothe phase conductor which has the absolutely greatest instantaneousnegative voltage value;

If the absolute value of the instantaneous current in a winding becomesequal to or greater than the threshold value,

and if the sampled instantaneous voltage between the two terminals ofthe relevant winding also becomes less than the sampled instantaneousvoltage between the two corresponding current conductors of theauxiliary power supply,

actuation of the switches for interrupting the existing connections ofthe two terminals of the relevant winding and for establishing aconnection of that terminal of the relevant winding which is positive ornegative at that instant to the corresponding positive or negativecurrent conductor of the auxiliary power supply,

and if, on the other hand, the sampled instantaneous voltage between thetwo terminals of the relevant winding also becomes equal to or greaterthan the sampled instantaneous voltage between the two correspondingcurrent conductors of the auxiliary power supply,

actuation of the switches for interrupting existing connections of thetwo terminals of the relevant winding and for establishing a connectionof these two terminals to the corresponding terminals of the said devicefor the conversion of energy.

Here, the conversion device substantially comprises the winding and theauxiliary power supply with the store for electric energy which isconnected thereto and, as already described, may be a capacitor or astorage battery. In this exemplary embodiment, the device 217 describedabove is preferably programmed to bring about the energy conversion onthe basis of the results of the samplings, determinations andcomparisons specified above by establishing a connection of thatterminal of the winding which is positive at that instant to thatcurrent conductor of the auxiliary power supply which is negative atthat instant and of that terminal of the winding which is negative atthat instant to that current conductor of the auxiliary power supplywhich is positive at that instant.

It will be evident to a person skilled in the art of the invention thatsome modifications to the process steps and also additional processsteps and further measures may be provided within the scope of theinvention without departing from the teaching of the invention as aresult.

If the store for electric energy is a storage battery, it may beexpedient to take measures not to overcharge the latter. For example,the charging of the storage battery may be monitored by the pick-ups 264for voltage and current at the current conductors 251 of the auxiliarypower supply 250 and by the microprocessor 217, and the group ofswitches 216 may be controlled by the microprocessor 217 in such a waythat the storage battery is not overcharged. In this context it may beexpedient to arrange for the group of switches 216 to be controlled bythe microprocessor 217 in such a way that no electric energy is fed intothe auxiliary power supply 250 if the absolute value of theinstantaneous voltage at the current conductors 251 of the auxiliarypower supply 250 becomes greater than a predetermined upper thresholdvalue, and no electric energy is drawn from the auxiliary power supply250 if the absolute value of the instantaneous voltage at the currentconductors 251 of the auxiliary power supply 250 becomes less than apredetermined lower threshold value.

What is claimed is:
 1. A device for the reversible storage of electricenergy by reversible conversion of the electric energy into kineticenergy in an electric motor, the electric energy being drawn from ana.c. power supply of predetermined frequency and fed back into the a.c.power supply, comprising:a brushless, electronically commutated pancakeelectric motor with permanent magnets arranged on a rotor thereof andcoreless windings arranged on a stator thereof, each of said windingsbeing provided with a pair of terminals, which are respectivelyconnectable, each via a respective controllable switch, to at least onephase conductor of the a.c. power supply, devices for repeated samplingof an instantaneous voltage at one phase conductor each with respect toa neutral conductor or another phase conductor of the a.c. power supply,devices for repeated sampling of an instantaneous voltage between theterminals of at least one said pair of terminals, devices for repeatedsampling of an instantaneous current in each winding, and a device forcomparing the sampled instantaneous voltages or currents with oneanother and with corresponding threshold values and for controlling saidswitches in dependence on the instantaneous result of the comparisons.2. A device according to claim 1, wherein:the terminals of each pair ofterminals are directly connectable to each other via a respectivecontrollable switch.
 3. A device according to claim 1, wherein:saiddevice for comparing the sampled instantaneous voltages with one anotherand with corresponding threshold values and for controlling saidswitches in dependence on the instantaneous result of the comparisonscomprises a microprocessor.
 4. A device according to claim 1, furtherincluding:an auxiliary power supply, said terminals being connectablevia a controllable switch to at least one current conductor of saidauxiliary power supply.
 5. A device according to claim 4, furthercomprising:a store for electricity, said auxiliary power supplycomprising at least two current conductors which are connected to eachother via said store for electric energy.
 6. A device according to claim5, wherein:said store for electric energy is one of a capacitor and astorage battery.
 7. A process for the reversible storage of electricenergy by reversible conversion comprising:sampling instantaneousvoltage at one phase conductor each of an a.c. power supply with respectto a neutral conductor or another phase conductor of the a.c. powersupply and determining which said phase conductor has an instantaneousvoltage having a greatest instantaneous positive or negative value inabsolute terms; sampling instantaneous voltage between the two terminalsof each pair of terminals of respective windings of a brushless,electronically commutated pancake electric motor with permanent magnetsarranged on a rotor thereof and coreless windings arranged on a statorthereof, each of said windings being provided with a pair of terminalswhich are respectively connectable, each via a respective controllableswitch to at least one phase conductor of the a.c. power supply, anddetermining which terminal of the pair of terminals is positive ornegative at that instant; sampling instantaneous current in each saidwinding and comparing an absolute value of this instantaneous currentwith a threshold value; if the absolute value of the instantaneouscurrent becomes less than the threshold value, actuating respective ofsaid switches for interrupting existing connections of the respectivetwo terminals of the corresponding said pair of terminals andestablishing a connection of the respective said terminal which ispositive at that instant, to the respective said phase conductor whichhas the greatest instantaneous positive voltage value in absolute terms,and the respective said terminal which is negative at that instant, tothe respective said phase conductor which has the absolutely greatestinstantaneous negative voltage value; if the absolute value of theinstantaneous current becomes equal to or greater than the thresholdvalue, respective of said switches for interrupting existing connectionsof the respective two terminals of the corresponding said pair ofterminals and establishing a connection of these two terminals tocorresponding terminals of a device, for converting energy stored insaid electric motor in kinetic form, into another form of energy; saidsampling taking place with a repetition frequency which is substantiallygreater than the frequency of the a.c. power supply.
 8. A processaccording to claim 7, wherein:said energy converting is brought about byconnecting the respective two terminals of the corresponding said pairof terminals.
 9. A process according to claim 8, wherein:the respectivetwo terminals are connected via a switch each to one and the same phaseconductor of the a.c. power supply.
 10. A process according to claim 8,wherein:the respective two terminals are connected via one switch eachto one phase conductor each of said a.c. power supply.
 11. A processaccording to claim 10, further comprising:determining an absolute valueof instantaneous voltage between pairs of phase conductors of said a.c.power supply, determining to which pair of phase conductors of said a.c.power supply at least absolute value of the instantaneous voltagecorresponds, and connecting one of each of the two respective terminalsto one phase conductor each of the pair of phase conductors thusdetermined.
 12. A process according to claim 11, wherein:the respectiveterminal which is positive at a respective instant is connected to therespective said phase conductor which is positive at said instant andthe respective terminal which is negative at said instant is connectedto the respective said phase conductor which is negative at saidinstant.
 13. A process according to claim 7, further comprising:samplinginstantaneous voltage between a pair of current conductors of anauxiliary power supply and comprising an absolute value of thisinstantaneous voltage with the absolute value of the instantaneousvoltage between the two respective terminals of a respective pair ofsaid terminals; if the absolute value of the instantaneous current in arespective winding becomes less than the threshold value, and if theabsolute value of the instantaneous voltage between the two respectiveterminals of the respective pair of said terminals becomes equal to orgreater than the absolute value of the instantaneous voltage between thetwo corresponding phase conductors of the a.c. power supply, actuatingthe respective said switches for interrupting existing connections ofthe two respective terminals of the corresponding said pair of terminalsand for establishing a connection of the respective terminal of thecorresponding said pair of terminals which is positive at said instantor negative at said instant to the correspondingly positive or negativecurrent conductor of said auxiliary power supply, and if, on the otherhand, the absolute value of the instantaneous voltage between the tworespective terminals of the pair of said terminals becomes less than theabsolute value of the instantaneous voltage between the twocorresponding phase conductors of said a.c. power supply, actuating therespective said switches for interrupting existing connections of therespective two terminals of the corresponding pair of said terminals andestablishing a connection of the respective terminal of thecorresponding pair of said terminals which is positive at said instantto the respective said phase conductor which has the greatestinstantaneous positive voltage value in absolute terms, and connectingthe respective said terminal of the corresponding pair of said terminalswhich is negative at said instant to the respective said phase conductorwhich has the greatest instantaneous negative voltage value in absoluteterms; upon the instantaneous voltage in a winding becoming equal to orgreater than said threshold value, and if the sampled instantaneousvoltage between the respective two terminals of a pair of terminalsbecomes less than the sampled instantaneous voltage between the twocorresponding current conductors of said auxiliary power supply,actuating the respective said switches for interrupting existingconnections of the respective two terminals of the corresponding pair ofsaid terminals and establishing a connection of the respective saidterminal of the corresponding pair of said terminals which is positiveor negative at said instant to the corresponding positive or negativecurrent conductor of said auxiliary power supply, and if, on the otherhand, the sampled instantaneous voltage between the respective twoterminals of a pair of said terminals becomes equal to or greater thanthe sampled instantaneous voltage between the two corresponding currentconductors of said auxiliary power supply, actuating the respective saidswitches for interrupting existing connections of the respective twoterminals of the corresponding pair of said terminals and establishing aconnection of these two terminals to the corresponding said terminals ofthe said motor.
 14. A process according to claim 13, wherein:saidconversion comprises: establishing a connection of the respective saidterminal which is positive at said instant to a respective currentconductor of said auxiliary power supply which is negative at saidinstant, and establishing a connection of the respective said terminalwhich is negative at said instant to the respective current conductor ofsaid auxiliary power supply which is positive at said instant.